Catalysts having alkoxide-modified supports and method of increasing the catalytic activity of a catalytic metal

ABSTRACT

A catalyst composition comprising a catalytic metal and a support, the support being prepared by depositing a metal alkoxide on a core support, then calcining the support.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of Application Ser. No. 655,991, filed Sept. 28,1984, now U.S. Pat. No. 4,559,364, which was a divisional of ApplicationSer. No. 567,112, filed Dec. 30, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to supported metal catalysts.

Catalytic metals play an important role in heterogeneous catalysis. Thecatalytic metals typically are employed on various support materials, asonly the surface of a metal particle can participate in a catalyticprocess. Many people have proposed various solutions to thelong-standing problem of how to disperse catalytic metals moreefficiently on the surface of a support material. However, the art hasnot recognized the present invention as being an improved solution tothe problem.

SUMMARY OF THE INVENTION

In one aspect, the present invention is an improved catalyst compositioncomprising a catalytic metal on an alkoxide-modified support.

In another aspect, the invention is a method of increasing the catalyticactivity of a catalytic metal, the method comprising supporting thecatalytic metal on a support prepared by contacting a metal alkoxidewith a core support, then calcining the mixture of the metal alkoxideand the core support thereby leaving a coating on the core support, thecoating comprising the metal oxide derived from the calcination of themetal alkoxide.

In yet another aspect, the present invention is the use of a catalystcomposition of the present invention in a process for producing methanein an improved yield by contacting carbon monoxide and hydrogen underreaction conditions.

Surprisingly, the supported catalyst composition of the presentinvention exhibits increased catalytic activity compared to supports nottreated by the method of this invention. Thus, the catalyst compositionof the present invention is useful in and catalytic process in whichenhanced activity of a supported metal catalyst is desirable. Forexample, the proper catalyst composition can be employed in theproduction of methane in improved yields.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst composition of the present invention has two requiredcomponents: a catalytic metal; and an alkoxide-modified support. For thepurposes of the present invention, the term "alkoxide-modified support"means a material prepared by depositing a thin layer of a metal alkoxideon a core support material and then converting the metal alkoxide to theoxide of said metal.

The alkoxide-modified support comprises a core support material havingon its outer surface a thin layer of a metal oxide produced from aprecursor metal alkoxide. The core support material can be any material,such as a refractory oxide, which will not decompose or melt whensubjected to calcination. Examples of typical core support materialsinclude alumina, zirconia, boria, thoria, magnesia, titania, tantala,chromia, silica, kieselguhr and mixtures of these materials. Thealuminas and silicas are preferred in view of their low cost. The coresupport material typically has a surface area in the region of about0.10 to 500 m² /g, preferably 20 to 200 m² /g, and most preferably over100 m² /g prior to the deposition of the metal alkoxide precursor. Thesesurface areas are as measured by the Brunauer-Emmett-Teller (BET)method. The BET method is described by R. B. Anderson, ExperimentalMethods in Catalytic Research, pp. 48-66, Academic Press, 1968.

The precursor metal alkoxide can be the alkoxide of almost any metal solong as said metal alkoxide will thermally decompose to form a metaloxide. Examples of preferred metals for use in the precursor metalalkoxide include the metals of Groups IIIA, IVA, IVB and VB of theperiodic table of the elements. Examples of typical precursor metalalkoxides include Al(OCH(CH₂ CH₃)(CH₃))₃, Ti(OCH(CH₃)₂)₄,Ta(OCH(CH₃)₂)₅, Si(OC₂ H₅)₄, Nb₂ (OC₂ H₅)10, Ta₂ (OC₂ H₅)₁₀, and thelike. Typically, the alkoxide moiety has from 1 to about 10 carbonatoms, preferably from about 2 to about 4 carbon atoms.

The alkoxide-modified support is prepared by techniques known in theart, e.g., incipient wetness impregnation techniques, etc. Metal oxideprecursors are deposited on the selected core support material followedby conversion into the oxide form by calcination. The alkoxide-modifiedsupport is prepared by impregnating the desired core support materialwith a solution of an alkoxide precursor of the desired metal oxide. Thesolution used in impregnating the core support material may be aqueousor organic, the only requirement being that an adequate amount ofprecursor compound for the selected metal oxide is soluble in thesolvent used in preparing the impregnating solution. Aqueous or alcoholsolutions, preferably aqueous or ethanol solutions, are normally usedfor convenience. When using the impregnaton technique the metal alkoxideimpregnating solution is contacted with the core support material for atime sufficient to deposit the metal alkoxide precursor material ontothe carrier either by selective adsorption or, alternatively, the excesssolvent may be evaporated during drying leaving behind the precursormetal alkoxide salt. Advantageously, the incipient wetness technique maybe used whereby just enough of a precursor metal alkoxide solution isadded to dampen and fill the pores of the powder of the above-recitedcore support material.

The composite thus prepared by any of the above-recited techniques, orby any other technique known in the art, is dried at a temperature offrom 50° to 300° C. to remove the excess solvent and then converted intothe oxide form by exposure at temperatures of from 150° to 800° C.,preferably 300°-700° C. in an atmosphere such as O₂, air, He, Ar orcombinations thereof. This exposure is for a time sufficient to convertessentially all of the metal alkoxide precursor into metal oxide. Thecalcination is useful to decompose the metal precursor to the oxideform. Calcination, however, may not be required for certain metalprecursors which readily convert into metal oxides.

The catalytic metal can be any metal or metal compound having catalyticactivity. Typical catalytic metals include the transition metals.Examples of preferred catalytic metals include the metals of Group VIIIof the periodic table of the elements, i.e., iron, ruthenium, osmium,cobalt, rhodium, iridium, nickel, palladium and platinum. The catalyticmetal is deposited on the alkoxide-modified support via methods known inthe art such as, for example, impregnation of the alkoxide-modifiedsupport with a salt of the catalytic metal. The salt of the catalyticmetal is converted to the metal by exposing the salt to a reducingatmosphere via methods known in the art. It is preferred to reduce thesalt of the catalytic metal in situ, i.e., while the salt is in thereaction vessel.

Typically, the precursor metal alkoxide is employed in an amountsufficient to result in a finished alkoxide-modified support which has,after calcination, at least a molecular monolayer of the metal oxide,formed from the metal alkoxide, covering the entire outer surface of thecore support material. Typically, the catalytic metal is employed in acatalytic amount. Preferably, the finished catalyst of the presentinvention will have a composition as follows: from about 0.1 to about 10weight percent catalytic metal; from about 0.1 to about 30 weightpercent of metal oxide from alkoxide precursor; and the remainder beingcore support material. More preferably, the finished catalyst of thepresent invention will have a composition as follows: from about 0.5 toabout 5 weight percent catalytic metal; from about 1 to about 5 weightpercent metal oxide from alkoxide precursor; and the remainder beingcore support material.

The catalyst composition of the present invention is useful in anyapplication in which enhanced activity of a supported catalytic metal isdesirable. The production of methane from CO and H₂ is an example of apreferred use of the catalyst composition of the present invention. Theart contains many examples of metals known to be useful in reactingcarbon monoxide with hydrogen to produce a variety of compounds,including hydrocarbons and oxygenated compounds. These metals include,among others, Mo, W, Rh, Ru, Re, Pd, Ni, Co, and Fe. In what has come tobe called the Fischer-Tropsch Synthesis, carbon monoxide and hydrogenare reacted over a metal catalyst to produce saturated and unsaturatedhydrocarbons and oxygenated compounds containing from 1 to as many as1000 carbon atoms. The hydrocarbons can be aliphatic, alicyclic, oraromatic. Commercial utilization of this synthesis prior to 1950 wasaccomplished largely in Germany and is summarized in Storch, Columbic,and Anderson: The Fischer-Tropsch and Related Synthesis, John Wiley andSons, New York 1951.

The major disadvantage in the prior art processes and catalysts is thatmost of them are not capable of selectively producing methane.Surprisingly, at least one catalyst of the present invention may be usedto produce methane selectively by contacting carbon monoxide andhydrogen in the presence of said catalyst under reaction conditions.

The carbon monoxide required for the process can be obtained from anycarbon source, such as from the degradation of coal. The molar ratio ofhydrogen to carbon monoxide ranges generally from at least about 0.1 toabout 10, and preferably is from about 1 to about 3.

Process reaction conditions can vary over a rather wide range. Thepressure can vary from at least about 1 psig up to about 1500 psig.Atmospheric pressure is preferred for convenience. The reactiontemperature typically ranges from at least about 200° C. to about 600°C. and preferably is from about 200° C. to about 300° C.

Ruthenium is the preferred catalytic metal for use in the production ofmethane via the process of the present invention.

The following examples and comparative experiments are given toillustrate the invention and should not be construed as limiting itsscope. All parts and percentages are by weight unless otherwiseindicated.

SPECIFIC EMBODIMENTS Preparation of Alkoxide-Modified SupportsPreparation 1

A solution 4.5 g of Al(OCH(CH₂ CH₃)(CH₃))₃, obtained from Alfa Products,a Division of Morton Thiokol, Inc., in 20 ml of hexanes is added to asuspension of 10 g of γ-Al₂ O₃ core support material, obtained fromStrem Chemicals, Inc., and having a BET surface area of 100 m² /g in 125ml of hexanes to form a mixture. The mixture is stirred for 2 hours atroom temperature under an inert atmosphere. Then the hexanes are removedunder vacuum to yield a white powder. The powder is calcined in air at450° C. for 15 hours to form an AlO_(x) /γ-Al₂ O₃ alkoxide-modifiedsupport having a BET surface area of 120 m² /g. The weight ratio ofAlO_(x) to γ-aluminum is approximately 0.05.

Preparation 2

The procedure of Preparation 1 is followed except that the precursormetal alkoxide is Ti(OCH(CH₃)₂)₄, and the final productalkoxide-modified support is TiO_(x) /γ-Al₂ O₃.

Preparation 3

The procedure of Preparation 1 is repeated except that the precursormetal alkoxide is Ta(OCH(CH₃)₂)₄, and the alkoxide-modified support isTaO_(x) on γ-Al₂ O₃.

Example 1 Catalyst Preparation

Five grams of the support of Preparation 1 are added to a 100 mlsolution of 0.64 g of RuCl₃.(1-3H₂ O) in H₂ O. The mixture is stirredfor an hour and the water is removed under vacuum with steam heat. Thesolids are dried overnight at 110° C. in air. Analysis indicates thatthe solids contain 5 weight percent Ru.

General Reaction Procedure

A 16-inch long piece of 9/16 inch tubing of type 316 stainless steel isemployed vertically as a reactor. The reactor is equipped with a meansfor temperature control, and has 1 g of catalyst held in place by quartzwool in the center of the reactor. The catalyst is reduced in situ at400° C. for 15 hours with hydrogen at 50 cc/min. Then the reactor iscooled to 300° C. in flowing hydrogen gas. Then a feed stream consistingof 2 moles hydrogen per mole of CO is fed to the reactor under apressure of 1 atmosphere (14.7 psig) at 100 cc/min (gas hourly spacevelocity=6000/hr). The product stream is analyzed using gaschromatographic methods capable of detecting C₁ -C₅ hydrocarbons, C₁ -C₅alcohols, H₂, CO, and CO₂.

Examples 2-5 and Comparative Experiments 1-4

The General Reaction Procedure is followed for each Example andComparative Experiment, and each run is conducted for a 24-hour period.The results of each run are summarized in Table I.

Example 2

The catalyst of Example 1 is employed.

Comparative Experiment 1

The catalyst is 5 weight percent Ru on the γ-Al₂ O₃ core supportmaterial of Preparation 1 with no alkoxide modification.

Example 3

The catalyst is 5 weight percent Ru on the support of Preparation 2.

Comparative Experiment 2

The catalyst is 5 weight percent Ru on TiO₂, the untreated TiO₂ having aBET surface area of 100 m² /g.

Example 4

The catalyst is 5 weight percent Ru on the support of Preparation 3.

Comparative Experiment 3

The catalyst is 5 weight percent Ru on Ta₂ O₅, the untreated Ta₂ O₅having a BET surface area of 5 m² /g.

                  TABLE I                                                         ______________________________________                                        Methanation Results with 5 Weight                                             Percent Ruthenium Catalysts                                                                                     Selectivity                                                      Conversion   to Methane                                  Run    Catalyst      of CO (mole %)                                                                             (mole %)                                    ______________________________________                                        Ex. 2  Ru/AlO.sub.x /γ--Al.sub.2 O.sub.3                                                     99           100                                         C.E. 1 Ru/γ--Al.sub.2 O.sub.3                                                                62            99                                         Ex. 3  Ru/TiO.sub.x /γ--Al.sub.2 O.sub.3                                                     98           100                                         C.E. 2 Ru/TiO.sub.2  58           100                                         Ex. 4  Ru/TaO.sub.x /γ--Al.sub.2 O.sub.3                                                     97           100                                         C.E. 3 Ru/Ta.sub.2 O.sub.5                                                                         17            37                                         ______________________________________                                    

The results summarized in Table I indicate that the catalyst of thepresent invention unexpectedly and significantly outperforms, underidentical conditions, conventional catalysts supported on materials usedas the core support material of the catalyst of the present invention.

Example 5 and Comparative Experiment 4

Example 2 and Comparative Experiment 1 are repeated except that thecatalyst has 1 weight percent ruthenium. The results are summarized inTable II.

                  TABLE II                                                        ______________________________________                                        Methanation with 1 Weight Percent                                             Ruthenium Catalyst                                                                                              Selectivity                                                      Conversion   to CH.sub.4                                 Run    Catalyst      of CO (mole %)                                                                             (mole %)                                    ______________________________________                                        Ex. 5  Ru/AlO.sub.x /γ--Al.sub.2 O.sub.3                                                     90           99                                          C.E. 4 Ru/γ--Al.sub.2 O.sub.3                                                                 3           81                                          ______________________________________                                    

Surprisingly, at the lower catalyst loading, 1 weight percent ruthenium,the catalyst of the present invention significantly outperforms theequivalent catalytic metal on the conventional support. Moresurprisingly, a loading of 1 weight percent ruthenium on an alkoxidemodified support outperforms a conventional catalyst having a loading of5 weight percent ruthenium (see Comparative Experiment 1).

As previously mentioned, the preceding examples serve only to illustratethe invention and its advantages, and they should not be interpreted aslimiting since further modifications of the disclosed invention will beapparent to those skilled in the art. All such modifications are deemedto be within the scope of the invention as defined by the followingclaims.

What is claimed is:
 1. A catalyst composition comprising a catalyticmetal and an alkoxide-modified support, which support comprises a coresupport material having (a) a surface area of from about 20 to 200 m²/gram; and having (b) on the outer surface thereof a metal oxideproduced from a precursor metal alkoxide.
 2. The composition of claim 1wherein the catalytic metal is a Group VIII metal.
 3. The composition ofclaim 1 wherein the core support is a metal oxide.
 4. The composition ofclaim 1 wherein the alkoxide anion of the metal alkoxide has from 1 toabout 10 carbon atoms.
 5. A catalyst composition consisting essentiallyof the composition of claim
 1. 6. The composition of claim 1 wherein thecatalytic metal is from about 0.1 to about 10 weight percent of thetotal composition.
 7. The composition of claim 6 wherein the metal oxidelayer prepared from the metal alkoxide is from about 0.1 to about 30weight percent of the total composition.
 8. The composition of claim 7wherein the core support material is silica or alumina, the catalyticmetal is from about 0.5 to about 5 weight percent of the composition,and the metal oxide from metal alkoxide is from about 1 to about 5weight percent of the composition.
 9. The composition of claim 1 whereinthe metal of the metal alkoxide is a metal of Group IIIA, IVA, IVB orVB.
 10. The composition of claim 1 wherein the alkoxide anion of themetal alkoxide has from about 2 to about 4 carbon atoms.
 11. Thecomposition of claim 1 wherein the metal of the metal alkoxide isaluminum, tantalum, or titanium and the catalytic metal is ruthenium.12. A method of increasing the catalytic activity of a catalytic metal,the method comprising supporting the catalytic metal on a supportprepared by contacting a metal alkoxide with a core support, whichsupport has a surface area of from about 20 to about 200 m² /g, thentreating the mixture of the metal alkoxide and the core support toremove the carbon atoms therefrom, thereby leaving a coating on the coresupport, the coating comprising the metal oxide from the metal alkoxide.13. The method of claim 12 wherein the treating step to remove thecarbon atoms is a step comprising calcination.
 14. The method of claim12 wherein the core support is an oxide of a metal of Group IIIA, IVA,IVB or VB.
 15. The method of claim 12 wherein the metal of the metalalkoxide is a metal of Group IIIA, IVA, IVB or VB.
 16. The method ofclaim 12 wherein the catalytic metal is a metal of Group VIII.